In the field of silver halide photographic light-sensitive materials (particularly photographic emulsions), techniques are required for more effectively enhancing the photographic sensitivity of light-sensitive materials.
Chemical sensitizing agents conventionally have been added to photographic emulsions to enhance the intrinsic sensitivity of silver halide, for example, by gold sensitization, and group VIII metal sensitization.
Further, various spectral sensitizing agents (for example, methine sensitizing dyes) have been added, alone or in combination, to emulsions for imparting spectral sensitivity in a desired wavelength region to silver halide.
It is also known to super-additively enhance spectral sensitivity by using a certain spectral sensitizing dye in combination with another spectral sensitizing dye or a colorless compound which itself does not have a spectral sensitizing effect (i.e., "supersensitization").
Examples of colorless compounds having a supersensitizing effect include sulfonic acid derivatives (described in U.S. Pat. Nos. 2,937,089 and 3,706,567), heterocyclic compounds (described in U.S. Pat. No. 3,615,613, Japanese Patent Publication No. 38408/73, U.S. Pat. No. 3,592,656, Japanese Patent Application (OPI) No. 81613/76 (U.S. Pat. No. 4,030,927) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), U.S. Pat. Nos. 3,592,654 and 3,615,633, Japanese Patent Application (OPI) Nos. 90323/75 and 104927/75), sulfur-containing compounds (described in U.S. Pat. No. 3,457,078, Japanese Patent Application (OPI) No. 77224/76 (U.S. Pat. No. 4,097,284), U.S. Pat. Nos. 3,458,318, 3,954,481, 3,506,443 and 4,232,118, German Pat. No. 1,447,577), quaternary ammonium salts (described in U.S. Pat. Nos. 2,271,623, 3,481,742 and 2,860,982), and polycyclic aromatic compounds (described in U.S. Pat. No. 3,575,869). However, some of these compounds have the undesirable effect of deteriorating the stability of the emulsion or increasing fog, and most of them have the disadvantage that their supersensitization effect is small.